Bifunctional Catalyst for Decomposition and Oxidation of Nitrogen Monoxide, Composite Catalyst Including the Same for Apparatus to Decrease Exhaust Gas, and Method for Preparation Thereof

ABSTRACT

Disclosed are a bifunctional catalyst for simultaneously removing nitrogen oxide and particulate matters, capable of decomposing nitrogen monoxide and generating nitrogen dioxide through oxidation of nitrogen monoxide, a composite catalyst including the catalyst for simultaneously removing nitrogen oxide and particulate matters used for an apparatus to decrease exhaust gas of diesel vehicles, and a method for preparation thereof. The catalyst and the composite catalyst can be used in a device for reducing exhaust gas contaminants mounted on a diesel vehicle and an exhaust gas purification system comprising the device.

RELATED APPLICATIONS

This application claims priority to Korean Patent Application No.10-2008-0126650, filed on Dec. 12, 2008, entitled, “Bi-functionalcatalyst for decomposing and oxidizing nitric oxide simultaneously andits preparation method therein”, which is incorporated herein byreference in its entirety; and also claims priority to Korean PatentApplication No. 10-2009-0038462, filed on Apr. 30, 2009, entitled,“Mixtured catalyst for emission reduction device of diesel vehicles andpreparing method for the same”, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a bifunctional catalyst forsimultaneously removing nitrogen oxide and particulate matters, capableof decomposing nitrogen monoxide and generating nitrogen dioxide throughoxidation of nitrogen monoxide, a composite catalyst including thecatalyst for simultaneously removing nitrogen oxide and particulatematters used for an apparatus to decrease exhaust gas of dieselvehicles, and a method for preparation thereof.

More particularly, the present invention relates to a bifunctionalcatalyst for simultaneously removing nitrogen oxide and particulatematters, which may enable generation of nitrogen dioxide and, at thesame time, decomposition of nitrogen monoxide and include a supportcontaining metal oxide as well as a composite active metal, that has aco-catalyst of metal or metal oxide loaded on top of the support and anactive metal of metal or metal oxide loaded on top of the co-catalyst; acomposite catalyst for an apparatus to decrease exhaust gas of dieselvehicles, which includes the bifunctional catalyst, beta-zeolite, aninorganic binder and a dispersant; and a method for preparation thereof.

BACKGROUND ART

In recent years, due to strict regulation for carbon dioxide (CO₂)exhaust emission in overall industries, a demand for fuel-efficient(that is, high fuel economy) vehicles has shown a tendency to increase.For this reason, compared to diesel engines or conventional gasolineengines, a demand for a vehicle equipped with a gas direct injection(GDI) type engine having excellent energy efficiency has tended toincrease. Comparing the diesel engine and GDI engine, when fuelcombustion occurs in an engine chamber, the combustion of the fuel iscarried out using more oxygen than is required in a theoretical air fuelratio, in turn increasing efficiency of combustion and improving fueleconomy. However, the foregoing entails disadvantages of highconcentration of nitrogen oxides which refer to both of nitrogenmonoxide (NO) and nitrogen dioxide (NO₂) (hereinafter, referred to as‘NO_(x)’. Since contaminants such as nitrogen oxide, particulatematters, etc., seriously affect human health, emission regulations ofnitrogen oxides and particulate matters have been strengthenedthroughout the world.

Specifically, a great effort has been made to remove NO_(x) as a primarycause of an increase in ozone concentration, destruction of the ozonelayer and acid rain in the lower atmosphere, and systems for treatmentof vehicle exhaust gas such as Lean NO_(x) Trap (LNT), a selectivecatalytic reduction (SCR), etc., are known to exhibit high NO_(x)decomposition efficiency. Among those, SCR includes a reductive reactionusing a reducing agent such as hydrogen carbide (HC), ammonia (NH₃),urea, etc., to reduce NO_(x) into nitrogen in a presence of a catalyst(see Equation 1). A flow charge of a system for post-treatment ofexhaust gas through the foregoing is shown in FIG. 1.

NO_(x)+HC(or urea)→N₂+CO₂+H₂O  Equation 1

As shown in FIG. 1, an un-combusted hydrogen carbide and carbon monoxidecontained in the exhaust gas emitted from an engine 100 are oxidized ona diesel oxidation catalyst 600, in turn being harmless. Particulatematters (PMs) are trapped by a diesel particulate filter 300 whilenitrogen oxide contained in the exhaust gas is subjected to reductivereaction on a selective reduction catalyst 500 as well as a reducingagent provided from a rear end of the filter, in turn being reduced intoN₂.

Here, an SCR catalyst using urea may be prepared and used by loading orion-exchanging an active metal, which consists of a noble metal and/or atransition metal, on a zeolite support (see JP 2008-212799, and WO2004/045766). Use of a composite oxide of titanium and tungsten as acatalyst support and use of an active metal selected from cerium,lanthanum, praseodymium, niobium, nickel and tin have been disclosed inU.S. Pat. No. 5,658,546. Regarding NO reduction of using hydrogencarbide (HC—SCR), it was reported that excellent performance can beattained by loading tungsten on Zr—Ti composite oxide and loading Pt onan outer surface thereof (see Japanese Patent laid-open No.2004-105964).

However, as shown in FIG. 1, a NO removing system using a reducing agentneeds a device for supplying the reducing agent and alternativereduction catalyst (SCR) 500 for removing NO_(x), thus incurringincreased cost of maintenance due to supply of the reducing agent aswell as initial investment costs.

On the other hand, if a catalyst for directly decomposing NO is used,problems encountered in the foregoing SCR system using the reducingagent, that is, installation of an additional system forstorage/provision of a reducing agent, control logic for driving system,increase in initial investment costs and wheeled transport costs, or thelike, may be overcome.

NO_(x) direct decomposition catalyst is used to decompose NO_(x) intonitrogen and oxygen without using alternative reducing agents andextensive studies into industrial applications thereof have currentlybeen conducted. According to such studies, it has been reported thattransition metal loaded zeolite or perovskite catalysts may exhibitactivity on NO direct decomposition.

However, since the foregoing catalyst is activated at a high temperatureof 500° C. or more, activity of the catalyst is too low to be employedin a catalyst system for removing exhaust gas having a distribution ofconsiderably low temperatures and the catalyst has insufficientdurability. In addition, due to a great amount of oxygen, moisture,sulfur, etc., contained in vehicle exhaust gas, the activity of thecatalyst is considerably decreased, in turn requiring somereinforcement.

A bifunctional catalyst according to the present invention has excellentefficiency of decomposing nitrogen oxides (NO_(x)) at 250 to 500° C.which is a distribution of temperatures for vehicle exhaust gas, nodecrease in activity depending upon reaction time, and superiordurability with regard to oxygen, moisture and sulfur. Furthermore, thebifunctional catalyst of the present invention may decompose nitrogenoxide, in particular, nitrogen monoxide (NO) and, at the same time,partially oxidize NO into NO₂ as a side product. When such NO₂ is fedinto a diesel filter at a rear end thereof, this gas may have animportant role in oxidation of PMs trapped in the filter.

In order to remove such PMs contained in the vehicle exhaust gas, mostrelated industries have currently adopted a process that passes exhaustgas through a filter system including at least one selected from a groupconsisting of silicon carbide (SiC), cordierite and metal to trap PMs inthe filter, in turn removing the same. In this case, as an amount of PMsaccumulated in the filter is increased, problems such as engine overloadmay be caused. Such accumulated PMs are oxidized/removed using anoxidizing agent and thermal energy. Here, a process for removing PMstrapped in the filter is generally referred to as ‘regeneration’.

In general, when oxygen is used as an oxidizing agent to oxidize PMstrapped in a filter, filter regeneration may be executed at atemperature of 500° C. or more. Since a probability for formation a hightemperature exhaust gas is extremely low under actual driving conditionsof vehicles, there is a need to employ a natural generation system usingan oxidizing agent having higher oxidation capability than oxygen inorder to oxidize PMs at a relatively low temperature, and a forcedregeneration system using a thermal energy supply device mounted on anouter side of the system to forcedly increase a temperature of theexhaust gas, thereby oxidizing PMs.

The latter, that is, the forced regeneration system requires a greatamount of energy to elevate a temperature of exhaust gas to aregeneration temperature of 500° C. or more, in other words, involvesexcessive consumption of fuel, and entails a problem of deterioration infuel economy due to repeated regeneration or increased pressure causedby PMs. Therefore, a systemic configuration using a better oxidizingagent than O₂ to oxidize PMs at a lower temperature is most suitable inview of operational costs.

As described above, when PMs trapped in the filter are oxidized by O₂,an oxidation initiating temperature is about 300° C., however, oxidationis not actively progressed until about 400° C. or more due to influenceof contents of O₂, moisture, sulfur and HC contained in exhaust gas. Onthe other hand, if NO₂ is used as an oxidizing agent, an oxidationinitiating temperature is about 100° C. and, since NO₂ is used tooxidize PMs, a filter regeneration temperature may be considerablydecreased. FIG. 2 schematically illustrates a flow chart of a filterregeneration system to oxidize and remove PMs using NO₂ as an oxidizingagent.

The process described above includes converting NO, which accounts formore than 90% of NO components in exhaust gas generated from the engine100, into NO₂ on a noble metal catalyst 600 (see the following Equation2) and inducing oxidation of PMs in a filter 300 by the generated NO₂(see the following Equation 3).

As described above, a continuous regeneration type exhaust gas treatmentsystem shown in FIG. 2 adopts a simple structure, does not need anadditional energy source and shows excellent thermal efficiency.However, for vehicles having the foregoing system, a coefficient of NOutilization in a conventional catalyst system is relatively low.Accordingly, the foregoing system should be applied to only vehiclesthat have NO_(X)/PM concentration ratio of at least 20 in the exhaustgas and at least 50% of a total driving area in which a temperature ofexhaust gas is 250° C. or more.

NO+½O₂→NO₂  Equation 2

NO₂+C(particulate matter)→N₂+NO+CO(or CO₂)  Equation 3

Meanwhile, vehicles having difficulty in applying the continuousregeneration type exhaust gas treatment system, e.g., a vehicle drivenat a low speed in urban areas must have a forced regeneration typedevice for post-treatment of exhaust gas shown in FIG. 3.

A significant feature of such a forced regeneration type exhaust gaspost-treatment system is to heat the exhaust gas generated in the engine100 to at least a regeneration temperature of 500° C. or more by aheater 400 for supplying thermal energy, in turn oxidizing PMs. Comparedto the continuous regeneration type system for treatment of exhaust gasshown in FIG. 2, the foregoing system encounters a problem of increasingmaintenance costs due to operation of the heater 400 to supply thermalenergy. In particular, if a regeneration cycle is short, maintenancecosts for heating the exhaust gas are considerably increased.Accordingly, there is a need to extend the regeneration cycle byapplying a continuous regeneration type catalyst system to shorten theregeneration cycle to an existing forced regeneration exhaust gassystem, in turn decreasing fuel consumption.

Extensive research and investigation into diesel particulate filtersassociated with post-treatment techniques, in order to comply withreinforced regulations for exhaust gas emission standards of dieselvehicles, has recently been conducted. In addition, studies intocomposite catalysts used in an apparatus for decreasing exhaust gasemission of diesel vehicles equipped with the foregoing dieselparticulate filter having improved efficiency of removing particulatematters, have actively been conducted.

DISCLOSURE Technical Problem

Therefore, the present invention is directed to solving problemsdescribed above and an object of the present invention is to provide acatalyst for simultaneously removing nitrogen oxide and particulatematters, based on bifunctional catalytic performance including nitrogenmonoxide (NO) decomposition and nitrogen dioxide (NO₂) generationthrough NO oxidation under exhaust gas conditions with high oxygenconcentration (>4% O₂), without using a reducing agent, whilecompensating defects of conventional exhaust gas post-treatmentcatalysts.

Another object of the present invention is to provide a method formanufacturing a catalyst capable of simultaneously removing nitrogenoxide and particulate matters, based on bifunctional catalyticperformance including NO decomposition and NO₂ generation through NOoxidation under exhaust gas conditions with high oxygen concentration(>4% O₂), without using a reducing agent, while compensating for defectsof conventional exhaust gas post-treatment catalysts.

Another object of the present invention is to provide a compositecatalyst for an exhaust gas reducing device mounted on a diesel vehicle,which is applied to the device to improve efficiency of oxidizingun-combustible hydrogen carbide, carbon monoxide, nitrogen oxide, PM(particulate matter in exhaust gas), which are harmful to the humanbody, as well as the collection efficiency of carbon nanoparticleshaving a size of 30 nm or less.

Another object of the present invention is to provide a method formanufacturing a composite catalyst for an exhaust gas reducing devicemounted on a diesel vehicle.

A still further object of the present invention is to provide an exhaustgas reducing device with improved capability of reducing nitrogen oxide,which contains a bifunctional catalyst for simultaneously removingnitrogen oxide and PM to enable NO decomposition and NO₂ generationthrough NO oxidation, or a composite catalyst for an exhaust gasreducing device mounted on a diesel vehicle, as well as an exhaust gaspurification system having the same.

Technical Solution

In order to accomplish the foregoing objects, according to an embodimentof the present invention, there is provided a bifunctional catalyst forsimultaneously removing nitrogen oxide and particulate matters (PMs) toenable nitrogen monoxide (NO) decomposition and nitrogen dioxide (NO₂)generation through NO oxidation, the bifunctional catalyst comprising: asupport containing oxides of at least one element selected from a groupconsisting of titanium (Ti), zirconium (Zr), silicon (Si), aluminum (Al)and cerium (Ce); and a composite active metal formed by loading aco-catalyst based on at least one metal selected from a group consistingof tungsten (W), molybdenum (Mo), cobalt (Co), manganese (Mn), copper(Cu) and iron (Fe) or metal oxides thereof on top of the support, andloading an active metal based on at least one metal selected from agroup consisting of platinum (Pt), palladium (Pd), rhodium (Rh),ruthenium (Ru) and silver (Ag) on top of the co-catalyst.

According to the present invention, the co-catalyst may be loaded in anamount of 0.1 to 30 wt. % relative to a total weight of the support,while the active metal may be loaded in an amount of 0.1 to 10 wt. %relative to a total weight of the support.

According to the present invention, the co-catalyst may be loaded on anouter surface of the active metal and, preferably, an amount of thecatalyst loaded on the support may range from 0.1 to 10 wt. % relativeto a total weight of the support.

According to the present invention, an average particle diameter of thesupport may be larger than that of the composite active metal. Sinceaverage particle diameters are different therebetween, if a compositecatalyst of the present invention is applied to an exhaust gas reducingdevice mounted on a diesel vehicle, a contact area between the compositecatalyst and exhaust gas may be increased.

As a result, the exhaust gas reducing device coated with the compositecatalyst mounted on the diesel vehicle may improve oxidation efficiencyof harmful materials such as PM (particulate matter in exhaust gas) andcollection efficiency of carbon nanoparticles having a size of 30 nm orless.

An average particle diameter of the support according to the presentinvention may range from 0.01 to 20 μm, preferably, 0.03 to 10 μm.

An average particle diameter of the composite active metal may rangefrom 1 to 100 nm, preferably, 3 to 20 nm.

In addition, the present invention provides a method for preparation ofa bifunctional catalyst for simultaneously removing nitrogen oxide andparticulate matters (PMs) to enable nitrogen monoxide (NO) decompositionand nitrogen dioxide (NO₂) generation through NO oxidation, the methodcomprising: (a) loading a co-catalyst based on at least one metalselected from a group consisting of tungsten (W), molybdenum (Mo),cobalt (Co), manganese (Mn), copper (Cu) and iron (Fe) or metal oxidesthereof on top of a support containing oxides of at least one elementselected from a group consisting of titanium (Ti), zirconium (Zr),aluminum (Al) and cerium (Ce); (b) loading an active metal based on atleast one metal selected from a group consisting of platinum (Pt),palladium (Pd), rhodium (Rh), ruthenium (Ru) and silver (Ag) or metaloxides thereof on top of the co-catalyst; and (c) drying, calcining andconducting reduction of the loaded materials after loading theco-catalyst and the active metal.

According to the present invention, the co-catalyst in step (a) may beloaded in an amount of 0.1 to 30 wt. % relative to a total weight of thesupport, and the active metal in step (b) may be loaded in an amount of0.1 to 10 wt. % relative to a total weight of the support.

In addition, the co-catalyst and the active metal may be simultaneouslyor sequentially loaded in step (c).

According to the present invention, step (c) may further comprise: aftersimultaneously or sequentially loading the co-catalyst and the activemetal and calcining the loaded materials to form a particulate catalyst,loading the co-catalyst on an outer surface of the active metal in thepresence of the particulate catalyst; and, after loading the co-catalyston the outer surface of the active metal, sequentially drying, calciningand conducting reduction of the loaded active metal. An amount of theco-catalyst loaded on the outer surface of the active metal may rangefrom 0.1 to 10 wt. % relative to a total weight of the support.

The drying may be conducted at 100 to 110° C. for 10 to 15 hours,preferably, at 105° C. for 12 hours.

The calcination may be conducted at 500 to 600° C. for 3 to 7 hours inan air atmosphere, preferably, at 550° C. for 5 hours in an airatmosphere.

The reduction may be conducted at 200 to 400° C. for 0.5 to 5 hours in ahydrogen atmosphere, preferably, at 300° C. for 1 hour in a hydrogenatmosphere.

According to the present invention, a bifunctional catalyst forsimultaneously removing nitrogen oxide and particulate matters, toenable decomposition of nitrogen monoxide (NO) and nitrogen dioxide(NO₂) generation through NO oxidation, may be prepared by the abovemethod.

A bifunctional catalyst for simultaneously removing nitrogen oxide andparticulate matters, which enables decomposition of NO and NO₂generation through NO oxidation, may be applied to a structural body toattain a decrease in an amount of catalyst to be used, ensuringmechanical stability and improvement of durability, etc. The structuralbody referred to herein is a monolith or foam type structural materialcomprising metal and inorganic materials. Any structural material towhich the inventive catalyst is applied to ensure favorable performanceof the catalyst may be used during applying the catalyst and features orconstructions of the structural body are not particularly limited.

A variety of methods for applying a catalyst to a structural body may beused.

For instance, the bifunctional catalyst prepared by the foregoing methodis treated by wet milling to prepare a catalyst slurry and, afterapplying the prepared slurry to a monolith, honeycomb or dieselparticulate filter (DPF) trap, the coated material is subjected todrying, calcining and reduction under the same conditions as those usedin preparation of powdery catalyst, as described above, to therebyobtain a coating catalyst formed on the monolith, honeycomb or DPF trap.When the formed catalyst is canned and provided to a vehicle, nitrogenoxide and particulate matters generated from the vehicle may besimultaneously removed (see FIG. 5). The foregoing coating method is anillustrative example of a method for coating a structural body with thebifunctional catalyst of the present invention, however, coatingprocedures or processes are not particularly limited in the presentinvention.

The present invention also provides a composite catalyst for an exhaustgas reducing device mounted on a diesel vehicle, which includes thecatalyst for simultaneously removing nitrogen oxide and particulatematters described above.

The composite catalyst for an exhaust gas reducing device according tothe present invention may include beta-zeolite, an inorganic binder anda dispersant.

The catalyst for simultaneously removing nitrogen oxide and particulatematters of the present invention may be contained in an amount of 5 to95 wt. % relative to a total weight of the composite catalyst.Preferably, the amount ranges from 30 to 60 wt. % and, more preferably,the amount ranges from 40 to 50 wt. %.

The inorganic binder used in the present invention may be any oneselected from a group consisting of alumina, titania and silicone. Anamount of the inorganic binder may range from 0.5 to 5 wt. % relative toa total weight of the composite catalyst.

The dispersant may be water or alcohol, without being particularlylimited thereto.

In addition, the present invention provides a method for preparation ofa composite catalyst for an exhaust gas reducing device mounted on adiesel vehicle, the method comprising: (a) loading a co-catalyst basedon at least one metal selected from a group consisting of tungsten (W),molybdenum (Mo), cobalt (Co), manganese (Mn), copper (Cu) and iron (Fe)or metal oxides thereof on top of a support containing oxides of atleast one element selected from a group consisting of titanium (Ti),zirconium (Zr), aluminum (Al) and cerium (Ce); (b) loading an activemetal based on at least one metal selected from a group consisting ofplatinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru) and silver(Ag) or metal oxides thereof on top of the co-catalyst; (c) drying,calcining and conducting reduction after loading the co-catalyst and theactive metal, to thereby obtain a catalyst powder; and (d) mixing thecatalyst powder with beta-zeolite, an inorganic binder and a dispersantto produce a composite catalyst.

According to the present invention, steps (a) to (c) of the foregoingmethod are substantially the same as described above.

In step (d), the catalyst powder may be added in an amount of 40 to 60wt. % relative to a total weight of the composite catalyst. Theinorganic binder may be any one selected from a group consisting ofalumina, titania and silicon, while the dispersant may be water oralcohol, without being particularly limited thereto.

The present invention also provides a device for reducing exhaust gascontaminants, comprising: the catalyst for simultaneously removingnitrogen oxide and particulate matters described above or the compositecatalyst for an exhaust gas reducing device described above.

According to the present invention, the device for reducing exhaust gascontaminants, may include: a catalyst coated honeycomb fabricated bycoating a honeycomb with the catalyst for simultaneously removingnitrogen oxide and particulate matters or the composite catalyst for anexhaust gas reducing device; and a filter, wherein the filter isconnected to the catalyst coated honeycomb.

According to the present invention, the device for reducing exhaust gascontaminants, may include: a catalyst coated honeycomb fabricated bycoating a honeycomb with the catalyst for simultaneously removingnitrogen oxide and particulate matters or the composite catalyst for anexhaust gas reducing device; and a filter for trapping particulatematters, wherein the filter is connected to the catalyst coatedhoneycomb.

According to the present invention, the device for reducing exhaust gascontaminants may include: a catalyst coated honeycomb fabricated bycoating a honeycomb with the catalyst for simultaneously removingnitrogen oxide and particulate matters or the composite catalyst for anexhaust gas reducing device; and a catalyst coated diesel particulatefilter (DPF) trap formed by coating an inner side of the DPF with thecatalyst for simultaneously removing nitrogen oxide and particulatematters or the composite catalyst for an exhaust gas reducing device,wherein the catalyst coated DPF trap is connected to the catalyst coatedhoneycomb.

Further, the present invention also provides an exhaust gas purificationsystem comprising the device for reducing exhaust gas contaminantsdescribed above.

According to the present invention, the exhaust gas purification systemmay further include a reducing agent supplying device.

An illustrative example of the exhaust gas purification system isschematically shown in FIG. 6. A catalyst enabling massive generation ofNO₂ as well as reduction of nitrogen oxide may be applied to a honeycombor monolith type support fabricated according to sequential orderillustrated in FIG. 5. Here, the honeycomb or monolith may consist ofceramic or metal.

With regard to construction of the system, exhaust gas emitted from anengine 100 is subjected to NO decomposition and, at the same time, NO₂generation on a surface of catalysts of a catalyst coated honeycomb 200,according to Equation 4. The generated NO₂ is reduced into N₂ or NOwhile oxidizing PMs trapped in a filter 300. According to this process,nitrogen oxide contained in the exhaust gas undergoes NO decompositionby the catalyst and generates NO₂ while decreasing an amount of thenitrogen oxide. The generated NO may be used as an oxidant for removingPMs, thereby continuously removing PMs trapped in the filter. In thiscase, the filter 300 may be any one consisting of ceramic or metal.

The exhaust gas purification system according to the present inventionmay also have an alternative construction as shown in FIG. 7.

The construction shown in FIG. 5 is applicable to an engine which emitsexhaust gases having a very high NO_(x)/PM ratio of 20 or more. However,if the NO_(x)/PM ratio is low, nitrogen oxide may be decomposed by thecatalysts of the catalyst coated honeycomb 200. Further, NO₂ selectivityis commonly 40% or less, thereby the above construction cannot provide asufficient amount of oxidant (NO₂) required for PM oxidation.Accordingly, a catalyst coated honeycomb may be fabricated by applyingthe inventive catalyst to an inner side of DPF 310, in particular, to asurface of honeycomb and used to improve utilization of NO (seeEquations 1 and 2 above). According to the fabricated honeycomb, whenthe DPF is exposed to a high temperature, PM contacting with thecatalyst may be directly oxidized (see Equation 4) and, at the sametime, NO reduced into an original condition by Equation 3 is againsubjected to reaction according to Equation 2, thus generating NO₂.Therefore, the catalyst coated honeycomb according to the presentinvention may enhance NO use efficiency, in turn increasing an amount ofPM to be removed.

C(PM)+O₂→CO₂(or CO)  Equation 4

The exhaust gas purification system according to the present inventionmay have an alternative construction shown in FIG. 8. According to theconstruction shown in FIG. 8, decomposition rate of nitrogen oxide maybe improved, compared to the construction shown in FIG. 7. About 10 to30% of NO among a total volume of NO_(x) contained in exhaust gasemitted from the engine 100 may be decomposed by the catalyst of thecatalyst coated honeycomb 200 to generate N₂. On the other hand, about10 to 40% of NO may be oxidized into NO₂. Since NO₂ is reduced into NOwhile oxidizing PM in the DPF 310, an amount of NO₂ remaining in theexhaust gas emitted from the DPF ranges from 65 to 85% relative to aninitial concentration of NO_(x).

The foregoing passes through a rear catalyst coated honeycomb 210, thusfurther decreasing nitrogen oxide by 10 to 30%. Consequently, a total NOdecomposition efficiency may become 20 to 50%, therefore, the aboveconstruction may be effective when it is applied to vehicles having highNO_(x)/PM ratio.

DESCRIPTION OF DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a view schematically illustrating a purification system for PMand nitrogen oxide;

FIG. 2 is a view schematically illustrating a continuous regenerationtype (CRT) exhaust gas purification system;

FIG. 3 is a view schematically illustrating a forced regeneration typeexhaust gas purification system;

FIG. 4 is a flow chart illustrating a process for preparation of apowder catalyst according to the present invention;

FIG. 5 is a flow chart illustrating a process for manufacturing a devicefor reducing exhaust gas and contaminants, used for vehicle test;

FIG. 6 is a view illustrating a configuration example 1 of an exhaustgas purification system according to the present invention;

FIG. 7 is a view illustrating a configuration example 2 of an exhaustgas purification system according to the present invention;

FIG. 8 is a view illustrating a configuration example 3 of an exhaustgas purification system according to the present invention;

FIGS. 9 and 10 shows test results of Examples 1 to 3 and ComparativeExample 1, especially, FIG. 9 shows NO_(x) decomposition efficienciesand FIG. 10 shows NO₂ generation efficiencies;

FIG. 11 is a photograph showing mounting of a catalyst/filter accordingto Example 4;

FIG. 12 shows vehicle driving data (vehicle speed, temperature ofexhaust gas, DOC+DPF differential pressure) of vehicle having thecatalyst of Example 1 coated therewith;

FIG. 13 illustrates a variation of PM accumulation depending uponvehicle driving;

FIG. 14 is a schematic view illustrating a DOC support/ceramic filtercoated with a composite catalyst for an exhaust gas reducing device fora diesel vehicle according to the present invention;

FIG. 15 is an SEM image showing a surface of the DOC support/ceramicfilter coated with a composite catalyst prepared in Example 5 of thepresent invention;

FIG. 16 is an SEM image showing a cross-section of the DOCsupport/ceramic filter coated with a composite catalyst prepared inExample 5 of the present invention;

FIG. 17 is a schematic view showing a DOC support/ceramic filter coatedwith Pt—W/TiO2 prepared in Example 6 of the present invention;

FIG. 18 is an SEM image showing a surface of a DOC support/ceramicfilter coated with Pt—W/TiO2 prepared in Example 6 of the presentinvention; and

FIG. 19 is an SEM image showing a cross-section of a DOC support/ceramicfilter coated with Pt—W/TiO2 prepared in Example 6 of the presentinvention.

DESCRIPTION OF SYMBOLS FOR MAJOR PARTS IN DRAWINGS

-   -   100: Engine, 200: Catalyst coated honeycomb    -   210: Rear catalyst coated honeycomb, 300: Filter    -   310: DPF, 400: Heater    -   500: SCR catalyst, 600: Diesel oxidation catalyst coated        monolith

BEST MODE

Exemplary embodiments of the present invention will be described indetail according to the following examples. However, the scope andspirit of the present invention disclosed in the appended claims are notrestricted to the foregoing exemplary embodiments but include variationsand/or equivalents of technical configurations of the invention.

Example 1

A powder catalyst according to the present invention was prepared by thefollowing procedures.

Titanium dioxide (TiO₂) powder was loaded in a water soluble solutioncontaining an active metal and a co-catalyst component dissolved thereinby an incipient-wetness method. Here, the used active metal andco-catalyst component were platinum (H₂PtCl₆.xH₂O, Aldrich Co.) andtungsten, respectively, individual precursors of these components weredissolved in distilled water such that contents of the loaded platinumand tungsten (Ammonium Tungstate, Aldrich Co.) became 2.0 wt. % and 5.0wt. %, respectively, relative to a total weight of a support.

Thereafter, a catalyst component containing platinum and tungsten loadedtherein was dried at 105° C. for 12 hours in an air atmosphere andcalcined at 550° C. in an air atmosphere. The calcined product wasmilled and subjected to measurement of NO_(x) decomposition performance.The catalyst was indicated as KOC-1.

For KOC-1 catalyst prepared as described above, after conductingreduction at 300° C. for 30 minutes using a reductant gas (10 vol %H₂/N₂), NO_(x) decomposition experiments were progressed. For NO_(x)decomposition efficiency and NO₂ generation efficiency were examinedunder conditions of 12.5% oxygen, 300 ppm NOx, 5% moisture andGHSV=50,000/hr, which are similar to exhaust gas conditions of lean burnvehicles. Such examination results are shown in FIGS. 9 and 10. FIG. 9shows NO_(x) decomposition efficiency and FIG. 10 shows NO₂ generationefficiency.

As a result of experiments, it was found that NO_(X) decompositioncapability and selectivity to NO₂ generation were considerably improved(in a range of 200 to 450° C.), compared to test results ofPt[5]/γ—Al2O3 (Comparative Example 1) generally used as an dieseloxidation catalyst (DOC) for exhaust gas purification of existing dieselengine automobiles.

In this regard, NO_(x) removal rate may be calculated by the followingmathematical equation 1 while NO₂ selectivity may be estimated by thefollowing mathematical equation 2.

NO_(X) removal rate=[concentration of NO_(x) emitted from catalystlayer/concentration of NO_(x) introduced into catalyst layer]×100  MathEquation 1

NO₂ selectivity=[concentration of NO₂ generated in catalystlayer/concentration of NO introduced into catalyst layer]×100  MathEquation 2

Example 2

A catalyst was prepared by the same procedure described in Example 1,except that ZrO₂ was used as a support of the catalyst (referred to asKOC-2).

For KOC-2 catalyst prepared as described above, after conductingreduction at 300° C. for 30 minutes using a reductant gas (10 vol %,H₂/N₂) and before conducting NO_(x) decomposition experiments,performance of the catalyst was evaluated. FIG. 9 illustrates NO_(x)decomposition efficiency while FIG. 10 shows NO₂ generation efficiency.

As a result of determining catalyst activity, it can be seen that NO_(x)decomposition capability was greatly improved as compared toPt[5]/γ—Al2O3, and NO_(x) decomposition capability and NO₂ generationselectivity were greatly improved as compared to commercially availablecatalysts.

Example 3

Pt[2]-W[5]/TiO₂ was prepared by loading, drying and calcining activemetal and co-catalyst according to the same procedures described inExample 1. In order to improve NO_(x) decomposition capability anddurability, tungsten (W) among a second group of co-catalysts wasadditionally loaded in an amount of 1.0 wt. % relative to a total weightof the support. Then, drying, calcining and reduction were conducted toprepare a catalyst. Such prepared catalyst was indicated to as KOC-3.

For KOC-3 catalyst prepared as described above, after conductingreduction at 300° C. for 30 minutes using a reductant gas (10 vol %,H₂/N₂) and before conducting NO_(x) decomposition experiments, activityof the catalyst was evaluated. FIG. 9 illustrates NO_(x) decompositionefficiency while FIG. 10 shows NO₂ generation efficiency.

As a result of determining catalyst activity, it can be seen that NO_(x)decomposition capability and NO₂ generation selectivity were greatlyimproved as compared to Pt[5]/γ—Al2O3 and KOC-1.

Example 4

A slurry solution was prepared by wet milling the catalyst KOC-1 powderaccording to Example 1. Ceramic monolith (400 cpi) was immersed into theslurry solution to coat a surface of the monolith with catalystcomponent. Immersion and drying were repeated until an amount of thecatalyst coating reached 60 g/L. After drying, the coated monolith wassubjected to calcination at 550° C. for 4 hours in an air atmosphere,then, reduction at 300° C. for 1 hour in a 10 vol % hydrogen/nitrogenatmosphere, thereby forming a DOC.

By combining the completed DOC (diameter of 14 cm, length of 7.3 cm, 400cpi) with ceramic DPF (diameter of 14 cm, length of 23 cm, 200 cpi), anintegrated can was fabricated and used to manufacture a contaminantreducing device.

The exhaust gas reducing device was mounted on an automobile, forexample, commercially available under the trade mane CARNIVAL (with TCIengine, KIA Motors, Korea) (see FIG. 11) and PM trapping amountdepending upon time was measured.

When the above automobile was driven with an average driving speed of 60km/hr or less (see FIG. 12), weight of a filter was measured at aconstant interval to estimate the PM trapping amount. Measured resultsare shown in FIG. 13.

In general, for a diesel vehicle equipped with a forced regenerationsystem, PM accumulation in DPF is proposed to be 5 g/L (20 g/4 L DPF).The reason for this is that DPF may be damaged by thermal energy givenfrom the forced regeneration system as well as thermal energy generatedby PM oxidation, if an amount of PM accumulation exceeds the abovelevel.

With regard to the diesel vehicle having with the inventive catalyst, PMaccumulation was measured. As a result, it was found that PMaccumulation per hour was decreased to 50%, as compared to a controlpart having DOC/cDPF (a catalyst in Comparative Example 1 below). Thismeans that, when 20 g of PM was accumulated in DPF and the forcedregeneration system was operated, a system having commercially availableDOC/cDPF (Pt[5]/γ—Al2O3) had to be periodically regenerated every 4hours while a system using KOC-1 catalyst of the present inventionenabled a regeneration period to be extended to 8 hours.

Accordingly, as shown in FIG. 2, if an exhaust gas purificationapparatus having a forced regeneration device is used, fuel consumptionmay be decreased to 50% or less. Specifically, as the regenerationperiod is extended as described above, lifespan of an air compressor, afuel pump, a battery, a fuel feeding valve, etc. may also be extended.

Comparative Example 1

An oxidation catalyst Pt[5]/γ—Al2O3, commercially available in the artwas prepared by the same procedures described in Example 1. Then, underthe same conditions as described in Example 1, catalyst activity wasmeasured.

Here, a support of the catalyst was γAl2O3 and, as an active ingredientof the catalyst, Pt was used in an amount of 5 wt. % relative to a totalweight of the support.

Comparative Example 2

The catalyst prepared in Comparative Example 1 was applied to a ceramichoneycomb and a filter (DPF; diameter of 14 cm, length of 23 cm, 200cpi) by the same procedures described in Example 4, to thereby completeDOC/cDPF. Performance of the completed DOC/cDPF was determined. In thiscase, a catalyst coating amount on the filter was 20 g/L and drying,calcining and reduction were conducted by the same process as that usedfor preparation of DOC.

A result of the determination is shown in FIG. 13. PM trapping amount ofDOC/cDPF was calculated by measuring difference in weights at apredetermined time interval during urban driving at 40 km/hr (◯), urbandriving at 60 km/hr (Δ), country road driving at 80 km/hr (∇) andhighway driving at 100 km/hr (□), respectively.

As a result, it was found that a time required to reach 20 g of PMaccumulation is 4 hours regardless of driving patterns. Although whenDPF was coated with the catalyst, PM accumulation was about 2 times asthat in Example 4.

From the above description, it can be understood that ‘DOC/cDPF’ coatedwith an existing oxidation catalyst commercially available in the marketcannot be employed in vehicles having relatively low exhaust gastemperature. Moreover, when the foregoing catalyst is applied to aforced regeneration system, a problem of increasing fuel consumption maybe expected.

Example 5

The powder catalyst prepared in Example 1, beta-zeolite (45 wt. %)having an average particle diameter of 400 nm and alumina sol (5 wt. %)as a binder were mixed together, followed by wet milling, in turnpreparing a composite catalyst for an exhaust gas reducing device for adiesel vehicle.

Example 6

In this example, the composite catalyst for an exhaust gas reducingdevice for a diesel vehicle prepared in Example 5 according to thepresent invention was coated with DOC/cDPF, and subjected to drying,calcining and reduction by the same procedures described in Example 4.The composite catalyst was applied in amounts of 60 g/L and 20 g/L toDOC and DPF, respectively.

As a result, DOC/cDPF coated with the composite catalyst of the presentinvention was obtained. FIG. 14 is a schematic view showing the coatedDOC/cDPF. As shown in FIG. 14, it can be seen that the DOC/cDPF coatedwith the inventive composite catalyst has the composite catalyst with asmall particle diameter uniformly distributed throughout an outersurface of beta-zeolite having a relatively large particle diameter.

FIG. 15 is an SEM image showing a surface of DOC coated with thecomposite catalyst of the present invention, while FIG. 16 is an SEMimage showing a cross-section of DOC coated with the composite catalystof the present invention.

As shown in FIGS. 15 and 16, beta-zeolite having a large particlediameter comprises a porous structure and the composite catalyst of thepresent invention is uniformly distributed throughout an outer surfaceof the beta-zeolite, thereby confirming that a catalyst area capable ofreacting with exhaust gas of the diesel vehicle is relatively large.

PM removal efficiency of DOC/cDPF was determined by the same proceduresdescribed in Example 4. However, experimental conditions were twodifferent modes of 60 km/hr and 100 km/hr, respectively.

Results of the experiments are shown in TABLE 1

As shown in TABLE 1, a PM accumulation rate where DOC/cDPF coated withthe composite catalyst of the present invention is used, was 1.0 g/hr ata low speed mode of 60 km/hr while being −6.0 g/hr at a high speed modeof 100 km/hr. On the other hand, if DOC/cDPF in Comparative Example,that is, a control is used, it can be seen that PM accumulation ratedemonstrates excellent driving efficiency.

TABLE 1 Comparison of catalyst performance PM accumulation PM removalSection Driving mode rate (g/hr) efficiency (%) DOC/cDPF in  60 km/hr  1.0  77.8 Example 6 100 km/hr −6.0 230.0 DOC/cDPF in  60 km/hr   2.0 55.5 Example 7 100 km/hr −2.0 144.0 Control  60 km/hr   4.5 —(Comparative 100 km/hr   4.5 — Example 2)

Example 7

In this example, DOC/cDPF was coated using Pt—W/TiO2 proposed in Example4 and according to the same procedure described in Example 6. However, abinder was added to Pt—W/TiO2 component without using beta-zeolite.

FIG. 17 is a schematic view illustrating the foregoing DOC/cDPF.

As shown in this schematic view, DOC/cDPF was coated with Pt—W/TiO2 as afine catalyst having a uniform particle diameter, thereby confirmingthat a surface area of the catalyst capable of reacting with exhaust gasof a diesel vehicle is relatively small.

FIG. 18 is an SEM image showing a surface of the coated DOC, while FIG.19 is an SEM image showing a cross-section of the coated DOC.

As shown in FIGS. 18 and 19, it can be seen that, when only Pt—W/TiO2having a fine particle diameter is applied to DOC/cDPF, porosity of thecatalyst Pt—W/TiO2 layer is low, thus causing a problem in contactbetween the catalyst and exhaust gas of a vehicle.

Performance of DOC/cDPF was determined by the same procedure describedin Example 6.

TABLE 1 shows results of the experiment.

Compared to zeolite-free DOC/cDPF (Example 6), activity was relativelylow. However, the activity was remarkably improved, as compared toresults of a control (Comparative Example 2).

Although preferred embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various alterations and modification are possible,without departing from the scope and spirit of the present invention asdisclosed in the appended claims.

INDUSTRIAL APPLICABILITY

According to the present invention, a bifunctional catalyst forsimultaneously expressing activities in relation to NO directdecomposition and NO₂ generation or a composite catalyst for an exhaustgas reducing device for a diesel vehicle which includes a catalyst forsimultaneously removing nitrogen oxide and particulate matters have beendeveloped and used to fabricate an exhaust gas post-treatment system.According to the foregoing, an exhaust gas purification system thatdecreases nitrogen oxide without using an alternative reducing agentand, at the same time, enables PM trapped in a filter to be decreasedeven under conditions of low exhaust gas emission may be provided.

If a bifunctional catalyst simultaneously expressing high activities inrelation to NO direct decomposition and NO₂ generation or a compositecatalyst according to the present invention is associated with existingSCR catalyst system, an improved exhaust gas purification system thatminimizes an amount of a reducing agent to be supplied and, at the sametime, maximizes efficiency thereof, may be provided.

Moreover, when the inventive catalyst is associated with a forcedregeneration system operated by a heat source, a long regenerationperiod may be applied, as compared to existing systems. Therefore, apost-treatment apparatus having excellent thermal efficiency may beprovided and, at the same time, nitrogen oxide may partially undergodirect decomposition.

1. A bifunctional catalyst for simultaneously removing nitrogen oxideand particulate matters (PMs) to enable nitrogen monoxide (NO)decomposition and nitrogen dioxide (NO₂) generation through NOoxidation, the bifunctional catalyst comprising: a support containingoxides of at least one element selected from a group consisting oftitanium (Ti), zirconium (Zr), silicon (Si), aluminum (Al) and cerium(Ce); and a composite active metal formed by loading a co-catalyst basedon at least one metal selected from a group consisting of tungsten (W),molybdenum (Mo), cobalt (Co), manganese (Mn), copper (Cu) and iron (Fe)or metal oxides thereof on top of the support, and loading an activemetal based on at least one metal selected from a group consisting ofplatinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru) and silver(Ag) on top of the co-catalyst.
 2. The bifunctional catalyst accordingto claim 1, wherein the co-catalyst is loaded in an amount of 0.1 to 30wt. % relative to a total weight of the support, and the active metal isloaded in an amount of 0.1 to 20 wt. % relative to a total weight of thesupport.
 3. The bifunctional catalyst according to claim 1, wherein theco-catalyst is loaded on an outer surface of the active metal.
 4. Thebifunctional catalyst according to claim 3, wherein the co-catalyst isloaded on the outer surface of the active metal in an amount of 0.1 to 5wt. % relative to a total weight of the support.
 5. The bifunctionalcatalyst according to claim 1, wherein an average particle diameter ofthe support is larger than that of the composite active metal.
 6. Thebifunctional catalyst according to claim 5, wherein the average particlediameter of the support ranges from 0.02 to 10 μm.
 7. The bifunctionalcatalyst according to claim 5, wherein the average particle diameter ofthe composite active metal ranges from 0.001 to 0.1 μm.
 8. A method forpreparation of a bifunctional catalyst for simultaneously removingnitrogen oxide and particulate matters (PMs) to enable nitrogen monoxide(NO) decomposition and nitrogen dioxide (NO₂) generation through NOoxidation, the method comprising: (a) loading a co-catalyst based on atleast one metal selected from a group consisting of tungsten (W),molybdenum (Mo), cobalt (Co), manganese (Mn), copper (Cu) and iron (Fe)or metal oxides thereof on top of a support containing oxides of atleast one element selected from a group consisting of titanium (Ti),zirconium (Zr), aluminum (Al) and cerium (Ce); (b) loading an activemetal based on at least one metal selected from a group consisting ofplatinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru) and silver(Ag) or metal oxides thereof on top of the co-catalyst; and (c) drying,calcining and conducting reduction of the loaded materials after loadingthe co-catalyst and the active metal.
 9. The method for preparation of abifunctional catalyst according to claim 8, wherein the co-catalyst instep (a) is loaded in an amount of 0.1 to 20 wt. % relative to a totalweight of the support, and the active metal in step (b) is loaded in anamount of 0.1 to 10 wt. % relative to a total weight of the support. 10.The method for preparation of a bifunctional catalyst according to claim8, wherein the co-catalyst and the active metal are simultaneously orsequentially loaded in step (c).
 11. The method for preparation of abifunctional catalyst according to claim 8, wherein step (c) furthercomprises: after simultaneously or sequentially loading the co-catalystand the active metal and calcining the loaded materials to form aparticulate catalyst, loading the co-catalyst on an outer surface of theactive metal in the presence of the particulate catalyst; and, afterloading the co-catalyst on the outer surface of the active metal,sequentially drying, calcining and conducting reduction of the loadedactive metal.
 12. The method for preparation of a bifunctional catalystaccording to claim 11, wherein the co-catalyst is loaded on the outersurface of the active metal in an amount of 0.1 to 10 wt. % relative toa total weight of the support.
 13. A composite catalyst for an exhaustgas reducing device mounted on a diesel vehicle, comprising. thecatalyst for simultaneously removing nitrogen oxide and particulatematters as set forth in claim
 1. 14. The composite catalyst according toclaim 13, further comprising beta-zeolite, an inorganic binder and adispersant.
 15. The composite catalyst according to claim 13, whereinthe catalyst for simultaneously removing nitrogen oxide and particulatematters is contained in an amount of 30 to 95 wt. % relative to a totalweight of the composite catalyst.
 16. The composite catalyst accordingto claim 14, wherein the inorganic binder is any one selected from agroup consisting of alumina, titania and silicone, and an amount of theinorganic binder ranges from 0.5 to 5 wt. % relative to a total weightof the composite catalyst.
 17. The composite catalyst according to claim14, wherein the dispersant is water or alcohol.
 18. A method forpreparation of a composite catalyst for an exhaust gas reducing devicemounted on a diesel vehicle, the method comprising: (a) loading aco-catalyst based on at least one metal selected from a group consistingof tungsten (W), molybdenum (Mo), cobalt (Co), manganese (Mn), copper(Cu) and iron (Fe) or metal oxides thereof on top of a supportcontaining oxides of at least one element selected from a groupconsisting of titanium (Ti), zirconium (Zr), aluminum (Al) and cerium(Ce); (b) loading an active metal based on at least one metal selectedfrom a group consisting of platinum (Pt), palladium (Pd), rhodium (Rh),ruthenium (Ru) and silver (Ag) or metal oxides thereof on top of theco-catalyst; (c) drying, calcining and conducting reduction afterloading the co-catalyst and the active metal, to thereby obtain acatalyst powder; and (d) mixing the catalyst powder with beta-zeolite,an inorganic binder and a dispersant to produce a composite catalyst.19. The method for preparation of a composite catalyst according toclaim 18, wherein the catalyst powder is added in an amount of 30 to 95wt. % relative to a total weight of the composite catalyst, theinorganic binder is any one selected from a group consisting of alumina,titania and silicon, and the dispersant is water or alcohol.
 20. Adevice for reducing exhaust gas contaminants, comprising. the catalystfor simultaneously removing nitrogen oxide and particulate matters asset forth in claim 1 forth in claim
 13. 21. The device for reducingexhaust gas contaminants according to claim 20, further comprising: acatalyst coated honeycomb fabricated by coating a honeycomb with thecatalyst for simultaneously removing nitrogen oxide and particulatematters or the composite catalyst for an exhaust gas reducing device;and a filter, wherein the filter is connected to the catalyst coatedhoneycomb.
 22. The device for reducing exhaust gas contaminantsaccording to claim 20, further comprising: a catalyst coated honeycombfabricated by coating a honeycomb with the catalyst for simultaneouslyremoving nitrogen oxide and particulate matters or the compositecatalyst for an exhaust gas reducing device; and a filter for trappingparticulate matters, wherein the filter is connected to the catalystcoated honeycomb.
 23. The device for reducing exhaust gas contaminantsaccording to claim 20, further comprising: a catalyst coated honeycombfabricated by coating a honeycomb with the catalyst for simultaneouslyremoving nitrogen oxide and particulate matters or the compositecatalyst for an exhaust gas reducing device; and a catalyst coateddiesel particulate filter (DPF) trap formed by coating an inner side ofthe DPF with the catalyst for simultaneously removing nitrogen oxide andparticulate matters or the composite catalyst for an exhaust gasreducing device, wherein the catalyst coated DPF trap is connected tothe catalyst coated honeycomb.
 24. An exhaust gas purification systemcomprising the device for reducing exhaust gas contaminants as set forthin claim
 20. 25. The exhaust gas purification system according to claim24, further comprising a reducing agent supplying device.
 26. A devicefor reducing exhaust gas contaminants, comprising. the compositecatalyst for an exhaust gas reducing device as set forth in claim 13.